One method for making shaped refractory articles involves preparing an aqueous slurry composed of a colloidal suspension of silica and bulk ceramic fibers. A vacuum is used to pull the slurry from its container and to deposit the same onto a porous mold used to form the shaped article. Excess liquid is withdrawn through the porous mold by the vacuum. The shaped article containing slurry solids and ceramic fibers is then removed from the mold and heated at very high temperatures, resulting in the precipitation of the silica in the form of finely-divided particles within the fibrous matrix.
A major drawback to this system for making shaped refractory articles resides in the fact that, as the porous mold becomes clogged with fibers, the vacuum becomes incapable of withdrawing all of the excess liquid from the mold. In addition, a large amount of the colloidal particles are initially withdrawn with the liquid. It is thus possible to have a nonhomogeneous shaped article with little or no colloidal particles on the surface thereof. However, immediately below the outer surface of the shaped article is material essentially rich in the solid constituents of the slurry. Accordingly, the resulting shaped article does not possess a substantially uniform density throughout and is therefore prone to irregular shrinkage upon drying or heating.
Freezecast processes have also been used to make refractory articles. The following are illustrative of such procedures.
Smith-Johannsen in the U.S. Pat. No. 3,177,161 uses a dispersion containing colloidal silica and a silicophilic material to form the desired structure, freezes the dispersion, permits the frozen structure to thaw, and then evaporates any remaining water from the structure by drying.
In U.S. Pat. No. 3,512,571, Phelps forms a refractory mold by freezing on a pattern, a slurry containing an aqueous colloidal sol and a powdered refractory material and then firing the frozen shape without prior thawing. The firing is carried out at a temperature of 1400.degree. to 1600.degree. F. If desired, Phelps may oven dry to a temperature of 200.degree. to 500.degree. F. before firing.
Roelofs, in U.S. Pat. No. 3,816,572, prepares a form using a composition, such as disclosed in U.S. Pat. No. 3,512,571 referred to above, freezes the same, takes special precautions to avoid frosting, and then fires the resulting form at a temperature of 1500.degree. to 1900.degree. F.
Downing, et al. in U.S. Pat. No. 3,885,005, use a refractory composition containing a silica sol and water to form a mold, freeze the molded product, remove the molded product from the mold, subject the frozen material to an elevated temperature to thaw the ice and to dry the water from the material in order to prevent water from reacting with the surface of the refractory body, and then fire the dried material at a temperature of 2000.degree. to 3500.degree. F.
Myles, in U.S. Pat. Nos. 4,174,331 and 4,248,752 discloses processes wherein he uses a composition containing a liquid vehicle, such as water, ceramic fiber, colloidal silica and an adhesion-enhancing agent to form a refractory shape, and then evaporates water therefrom.
In U.S. Pat. No. 4,246,209, Smith-Johannsen supercools a molded material made from an inorganic particulate or ceramic slurry containing colloidal sol to a temperature where it spontaneously nucleates the slurry, resulting in the formation of a very large number of ice crystals that are consequently very small, thus producing a structure that is uniform throughout. The structures are removed from the mold, thawed, dried in an oven at 120.degree. F. and then fired at a temperature of 1250.degree. F.
Weaver's process in U.S. Patent No. 4,341,725 forms a refractory article from a mixture of a liquid and a powder, freeze drying and then firing. In order to prevent the formation of large crystals of ice, the mixture used contains a hydrogen bond forming compound.
I have previously attempted to prepare refractory articles containing ceramic fibers and silica particles homogeneously throughout the article by a freezecast process where a saturated mat was heated to remove water contained therein, but such articles had stronger interior sections than surface sections, indicating less silica adjacent the surface than in the interior section. I have now found, however, that by forming a preform from a saturated mat and saturating the preform with a colloidal silica suspension, surprising properties are achieved and the surface sections of the mat have increased strength that enables use of the resultant refractory articles in situations where previous mats were not suitable.
It is an object of the present invention to provide a process for the preparation of refractory articles that contain ceramic fibers and silica particles substantially uniformly disposed throughout the article, both at the interior and the surface areas.